More Evidence for an Upper Limit on Vitamin D

Just like exercise, more isn't always better.

A couple of new studies on vitamin D have just come out, both pointing to the same conclusion: more isn't necessarily always better. I blogged about one such study last year; the two new ones show a similar pattern. Basically, the studies took a large group of people who'd had blood tests to measure levels of 25(OH)D, a marker of vitamin D status, waited to see who died or developed heart disease, then looked for patterns.

The figure to note here is that people with vitamin D levels higher than 90 nmol/L were apparently 13% more likely to die during the study period.

The second study, published in the American Journal of Medicine (press release here), looked at data from10,170 Americans over the age of 18, and followed them for an average of 3.8 years. Here's the hazard ratio for two outcomes: all-cause mortality (solid lines) and heart disease (dashed lines). (In each case, there are three different lines: the top and bottom represent "95% confidence intervals," basically showing the highest and lowest values consistent with the statistics.)

Note here that the values are in ng/mL rather than nmol/L. The study concludes that higher vitamin D levels offer protection against heart disease and death – but only up to a certain point:

However, once the serum levels of 25(OH)D rise above its population median of 21 ng/mL [52.5 nmol/L], it offers no statistically significant protection for all-cause or cardiovascular disease mortality.

This is far from the last word on vitamin D, and there continues to be a steady stream of studies showing the risks associated with low levels of vitamin D. But there's mounting evidence that – just like exercise! – there's a point beyond which the benefits of more vitamin D taper off and perhaps reverse.